Buggy code:
public interface KandoeBackendAPI { @ GET ( "api//selection-cards//{id}" ) Call < List < Card > > getSelectionCardsById ( @ Path ( "id" ) int id ) { } @ GET ( "api//selection-cards//by-subtheme//{id}" ) Call < List < Card > > getSelectionCardsBySubthemeId ( @ Path ( "id" ) int id ) { } @ POST ( "api//selection-cards" ) Call < Card > addCard ( @ Body Card card ) { } @ PATCH ( "api//sessions//{id}//select-cards" ) Call < Void > addCardsToSession ( @ Path ( "id" ) int id , @ Body List < Card > cards ) { } @ GET ( "api//cards//{id}//position//{sessionid}" ) Call < Integer > getPositionInSession ( @ Path ( "id" ) <START_BUG> int cardId , @ Path ( "sessionid" ) <END_BUG> int sessionId ) ; @ PATCH ( "api//sessions//{id}//join" ) Call < Void > addPlayerToSession ( @ Path ( "id" ) int id ) { } @ GET ( "api//accounts//{id}" ) Call < UserAccount > getUserById ( @ Path ( "id" ) int id ) { } @ POST ( "api//accounts" ) Call < UserAccount > postUser ( @ Body UserAccount account ) { } @ PATCH ( "api//accounts" ) Call < Void > changeAccount ( @ Body UserAccount account ) { } @ GET ( "api//accounts//by-auth0-user-id//{secret}" ) Call < UserAccount > getUserId ( @ Path ( "secret" ) String secret ) { } @ GET ( "api//sessions" ) Call < List < Session > > getAllSessions ( ) { } @ GET ( "api//sessions//{id}" ) Call < Session > getSessionById ( @ Path ( "id" ) int id ) { } @ GET ( "api//sessions//by-organisation//{id}" ) Call < List < Session > > getSessionsByOrganisation ( @ Path ( "id" ) int id ) { } @ GET ( "api//sessions//by-subtheme//{id}" ) Call < List < Session > > getSessionsBySubtheme ( @ Path ( "id" ) int id ) { } @ GET ( "api//verbose//sessions//{id}" ) Call < Session > getVerboseSessionById ( @ Path ( "id" ) int id ) { } @ GET ( "api//sessions//by-user//{id}" ) Call < List < Session > > getSessionsByUserId ( @ Path ( "id" ) int id ) { } @ PATCH ( "api//sessions//{sessionId}//level-up-card//{cardId}" ) Call < Void > levelUpCard ( @ Path ( "sessionId" ) int sessionId , @ Path ( "cardId" ) int cardId ) { } @ GET ( "api//verbose//organisations" ) Call < List < Organisation > > getOrganisationsVerbose ( ) { } @ GET ( "api//organisations//{id}" ) Call < Organisation > getOrganisationById ( @ Path ( "id" ) int id ) { } @ GET ( "api//organisations//by-organiser//{id}" ) Call < List < Organisation > > getOrganisationsByOrganiserId ( @ Path ( "id" ) int id ) { } @ GET ( "api//chat-messages" ) Call < List < ChatMessage > > getChatMessages ( ) { } @ GET ( "api//chat-messages//{id}" ) Call < ChatMessage > getChatById ( @ Path ( "id" ) int id ) { } @ POST ( "api//chat-messages" ) Call < Void > postChatMessages ( @ Body ChatMessage chatmessage ) { } @ GET ( "api//chat-messages//by-session//{id}" ) Call < List < ChatMessage > > getChatMessagesBySessionId ( @ Path ( "id" ) int id ) { } @ GET ( "api//themes" ) Call < List < Theme > > getThemes ( ) { } @ GET ( "api//themes//{id}" ) Call < Theme > getThemesById ( @ Path ( "id" ) int id ) { } @ GET ( "api//themes//by-organisation//{id}" ) Call < Theme > getThemeByOrganisationId ( @ Path ( "id" ) int id ) { } @ GET ( "api//subThemes" ) Call < List < SubTheme > > getSubThemes ( ) { } @ GET ( "api//subThemes//{id}" ) Call < SubTheme > getSubThemeById ( @ Path ( "id" ) int id ) { } @ GET ( "api//subthemes//by-theme//{id}" ) Call < List < SubTheme > > getSubthemesByThemeId ( @ Path ( "id" ) int id ) { } }

Fix:
int id , @ Path ( "sessionid" )